Field of the Invention
The invention relates to a switchable band-pass filter. In particular, the invention relates to a narrowband band-pass filter switchable over a broad frequency band with two filter elements comprising microstripline technology, suitable for use as a so-called cosite filter with a compact structure which is particularly suitable for mass production and robust in operation.
Discussion of the Background
Frequency-hopping methods are used to an increasing extent in radio communications technology. Accordingly, the carrier frequency of the transmission link alternates in a rapid sequence known to the transmitter and receiver. For the transmitters and receivers used, the demand for narrowband filters which are adjustable in frequency over the relevant frequency ranges is derived from this. These filters must achieve a rapid adjustability of the mid-frequency and, if possible, a digital switching with a fine frequency resolution derived from the application.
Moreover, mobile communications devices provide transmission and reception component groups in close proximity. In addition to this, a plurality of communications devices is often active in the environment within the same frequency band at the same time. Accordingly, it is advantageous if the same filter can be used as a reception filter for selecting the desired reception signal and filtering out interference acting via the reception antenna, and also as a transmission filter for reducing undesirable radiation. Such a filter is referred to as a cosite filter and must be additionally capable of transmitting a corresponding transmission power with low insertion loss and, on the one hand, filtering signals in the forward direction from amplifier to antenna and, on the other hand, in the return direction from antenna to receiver.
The prior art is the realization of such filters as double-circuit resonator filters comprising two high-quality resonators coupled via a network of capacitors and/or inductances. The frequency tuning is implemented via connectable capacitors by means of semiconductor diodes. In this context, the resonators are generally high-quality coil resonators (especially air coils). The frequency adjustment is implemented by means of PIN diodes of connectable capacitors. The combination of the high-quality coil resonators and the use of PIN diodes allows the transmission of correspondingly high transmission powers.
However, the structure described provides a series of disadvantages. The use of two coil resonators leads to large mechanical dimensions, heavy mass, corresponding disadvantages in the robustness of the filter with regard to vibration stress, and large dimensions of the filter component group as a whole. A filter component group realized on this technical basis leads to correspondingly unfavourable requirements for integration in mobile devices. Moreover, a band-pass filter with two coil resonators is associated with further disadvantages during manufacture, calibration and operation. On the one hand, a corresponding filter structure for automated manufacture in production lines designed for surface-mounted components (English: surface mounted devices, abbreviation: SMD) and re-flow soldering methods (English: reflow soldering) is not possible in a fully automated manner, but also requires manual manufacturing steps. Additionally, both coil resonators must be matched exactly with one another in a difficult calibration step, in order to achieve a corresponding, constantly flat filter transmission function within the passband. A tuneable band-pass filter realized in this manner is cost intensive to manufacture and structured in a complex manner.